This can be a result of one of two things:
Most common would be the download links have expired (or file only partially downloaded). When you click a download link from your order and go to the download page, the download links will expire in a few hours and result in this error. This is a security measure. To get new links, just close the download page and click the download link in your order again. You will be presented with a new download page with new links.
The second reason is that our web-monkey fell asleep at the wheel and didn't set up the download correctly on our system. In this case, and you continue to have problems downloading, please contact us and specify which download is causing the error.